Inspection system for array of microcircuit dies having redundant circuit patterns

ABSTRACT

An inspection system (10, 100) employs a Fourier transform lens (34, 120) and an inverse Fourier transform lens (54, 142) positioned along an optic axis (48, 144) to produce from an illuminated area of a patterned specimen wafer (12) a spatial frequency spectrum whose frequency components can be selectively filtered to produce an image pattern of defects in the illuminated area of the wafer. Depending on the optical component configuration of the inspection system, the filtering can be accomplished by a spatial filter of either the transmissive (50) or reflective (102) type. The lenses collect light diffracted by a wafer die (14) aligned with the optic axis and light diffracted by other wafer dies proximately located to such die. The inspection system is useful for inspecting only dies having many redundant circuit patterns. The filtered image strikes the surface of a two-dimensional photodetector array (58) which detects the presence of light corresponding to defects in only the illuminated on-axis wafer die. Inspection of all possible defects in the portions of the wafer surface having many redundant circuit patterns is accomplished by mounting the wafer onto a two-dimensional translation stage and moving the stage (40) so that the illuminated area continuously scans across the wafer surface from die to die until the desired portions of the wafer surface have been illuminated. The use of a time delay integration technique permits continuous stage movement and inspection of the wafer surface in a raster scan fashion.

TECHNICAL FIELD

The present invention relates to inspection systems for use in themanufacture of microcircuits and, in particular, to a real-time defectinspection system for use in the manufacture of microcircuits of thetype that includes an array of dies each having many redundant circuitpatterns.

BACKGROUND OF THE INVENTION

Two exemplary and very similar inspection systems for pattern defects inphotomasks employed in the large-scale manufacture of semiconductordevices and integrated circuits are described in U.S. Pat. Nos.4,000,949 of Watkins and 3,614,232 of Mathisen. The systems of Watkinsand Mathisen contemplate the simultaneous inspection of all of the dieson a photomask which contains a regular array of normally identical diesto detect the presence of nonperiodic defects, i.e., defects in one dienot identically repeated in the remaining dies of the array.

This task is accomplished by illuminating simultaneously all of the diesof a specimen photomask with collimated coherent light emanating from alaser to develop a composite diffraction pattern whose spatialdistribution is the combination of two components. The first componentis the interference pattern of the array of dies, and the secondcomponent is the interference pattern of a single die of the array. Thefirst and second components are sometimes called an inter-dieinterference pattern and an intra-die interference pattern,respectively. The light transmitted by the photomask strikes adouble-convex lens which distributes the light on a spatial filterpositioned a distance equal to one focal length behind the lens.

The spatial filter comprises a two-dimensional Fourier transform patternof a known error-free reference photomask against which the specimenphotomask is compared. The filter is opaque in the areas correspondingto spatial frequency components of the error-free Fourier transformpattern and is transparent in areas not included in the error-freeFourier transform pattern. Neither the Watkins patent nor the Mathisenpatent specifies the design parameters of the lens. The Mathisen patentstates only that the lens is of suitable numerical aperture andmagnification power to cover the area of the specimen photomask.

The spatial frequency components corresponding to the defects in thespecimen photomask are largely transmitted through the spatial filterand can be processed in either one of two ways. In the Watkins system,the light transmitted through the spatial filter strikes anotherdouble-convex lens that is properly positioned to define an image of thespecimen photomask, absent any information blocked by the spatialfilter. The imaging light not blocked by the spatial filter appears inlocations that represent the position in the specimen photomask wheredefects are present. In the Mathisen system, the light transmittedthrough the spatial filter is sensed by a photodetector that produces anoutput signal which activates a "no-go" alarm.

The Watkins and Mathisen patents imply that systems of the type theydescribe require both inter- and intra-die interference patterninformation to determine the presence of defects in the specimenpattern. The inter-die interference pattern information is of particularconcern because it consists of very closely spaced light spots that areextremely difficult to resolve by a Fourier transform lens. Therealization of such a lens is further complicated for inspection systemsthat use an inverse Fourier transform lens to form an image of thespecimen pattern from the Fourier transform light pattern. The reason isthat the design of each of the lenses is compromised to accomplish anoverall system design that accomplishes both the Fourier transformpattern and image forming functions. It is, therefore, exceedinglydifficult to obtain from such a system design the resolution required toacquire inter-die interference pattern information. The above lensdesign problem is encountered in systems of the type that simultaneouslyinspects the entire area of each of the dies of a specimen photomaskarray and, as a consequence, renders such systems unreliable andimpracticable for commercial use.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide a reliabledefect inspection system for use in the manufacture of microcircuits.

Another object of this invention is to provide such a system thatapplies the techniques of Fourier optics but does not contemplate theuse of inter-die interference pattern information to determine thepresence of defects in the manufacture of microcircuits of the type thatcomprises an array of normally identical dies.

A further object of this invention is to provide such a system that iscapable of developing from a microcircuit pattern an essentiallyaberration-free Fourier transform light pattern from which an accurateimage corresponding to defects in the microcircuit pattern can beformed.

Still another object of this invention is to provide an inspectionmethod that uses intra-die interference pattern information to determinethe presence of defects in a microcircuit array pattern of normallyidentical dies.

The present invention relates to a method and system for use in themanufacture of microcircuits and is described herein by way of exampleonly with reference to a real-time inspection system for defects insurfaces of semiconductor wafers of the type that includes an array ofcircuit dies of which each has many redundant circuit patterns. Suchsemiconductor wafers include, for example, random access and read onlymemory devices and digital multipliers.

Two preferred embodiments of the inspection system employ a Fouriertransform lens and an inverse Fourier transform lens positioned along anoptic axis to produce from an illuminated area of a patterned specimenwafer a spatial frequency spectrum whose frequency components can beselectively filtered to produce an image patter of defects in theilluminated area of the wafer. The lenses collect light diffracted by awafer die aligned with the optic axis and light diffracted by otherwafer dies proximally located to such die, rather than light diffractedby the entire wafer. This restriction limits the applicability of theinspection system to dies having many redundant circuit patterns butpermits the use of lenses that introduce off-axis aberrations that wouldotherwise alter the character of the Fourier transform pattern and thefiltered defect image.

Such lenses are relatively easy to manufacture because the redundantcircuit patterns typically repeat at 50 micron intervals and therebyproduce spatial frequency components spaced apart by a distance of about1.0 millimeter, which is resolvable by conventional optical components.The Fourier transform and imaging areas are preferably of sufficientsizes to accommodate light from only the wafer die aligned with theoptic axis. The spatial filter blocks the spatial frequencies of theerror-free Fourier transform of such die, i.e., the spatial filtercontains only intra-die interference pattern information.

The wafer is positioned in the front focal plane of the Fouriertransform lens, and the patterned surface of the wafer is illuminated bya collimated laser beam. The Fourier transform pattern of theilluminated wafer surface is formed in the back focal plane of theFourier transform lens. A previously fabricated spatial filter ispositioned in the plane of the Fourier transform pattern and effectivelystops the light transmission from the redundant circuit patterns of theilluminated dies of the wafer but allows the passage of lightoriginating from possible defects.

The inverse Fourier transform lens receives the light either transmittedthrough or reflected by the spatial filter and performs the inverseFourier transform on the filtered light diffracted by the illuminatedwafer area. Whether the spatial filter is of a type that transmits orreflects light depends on the embodiment of inspection system in whichit is incorporated. The filtered image strikes the surface of atwo-dimensional photodetector array which detects the presence of lightcorresponding to defects in only the illuminated on-axis wafer die. Thephotodetector array is centrally positioned about the optic axis and hasa light-sensitive surface area of insufficient size to cover the imageplane area in which the defect image corresponding to the on-axis dieappears. The inspection of all possible defects in the portions of thewafer surface having many redundant circuit patterns is accomplished bymounting the wafer onto a two-dimensional translation stage and movingthe stage so that the illumination area defined by the laser beamcontinuously scans across the wafer surface from die to die until thedesired portions of the wafer surface have been illuminated. The use ofa time delay integration technique permits continuous stage movement andinspection of the portions of the wafer surface having many redundantcircuit patterns in a stripe-to-stripe raster scan fashion.

The present invention is advantageous because the spatial filter neednot be fabricated with the use of an error-free specimen wafer. Thereason is that any defects present in such a wafer would produce lightof insufficient intensity to expose the spatial filter recording medium.

The present invention detects defects in a specimen pattern by usingonly intra-die information corresponding to areas of the specimenpattern having many redundant circuit patterns. The premises underlyingthe inspection method of the present invention are that inter-dieinterference pattern information is unnecessary if only areas of manyredundant patterns are inspected and that inspection of only such areasprovides sufficient statistical sampling to determine the defectdistribution for the entire specimen pattern.

Additional objects and advantages of the present invention will beapparent from the following detailed description of preferredembodiments thereof, which proceeds with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the optical components of a firstpreferred embodiment of the defect inspection system of the presentinvention.

FIG. 2 is a diagram of a semiconductor wafer comprising a regular arrayof normally identical dies of the type suitable for defect inspection bythe systems of FIGS. 1 and 6.

FIGS. 3A-3C are photographs of an exemplary single die of thesemiconductor wafer of FIG. 2 showing within such die a highly redundantcircuit pattern for consecutively increasing magnifications.

FIG. 4 is a simplified diagram showing the asymmetry of the Fouriertransform and inverse Fourier transform lens system incorporated in thedefect inspection system of FIG. 1.

FIG. 5 is a diagram showing the optical elements of the lens system ofFIG. 4.

FIG. 6 is a schematic diagram of the optical components of a secondpreferred embodiment of the defect inspection system of the presentinvention.

FIG. 7 is a cross sectional view of the spatial filter employed in thedefect inspection system of FIG. 6.

FIG. 8 shows the optical components of the Fourier transform and theinverse Fourier transform lens system incorporated in the defectinspection system of FIG. 6.

FIG. 9 is an isometric view of the scanning mechanism for detecting thepresence and locations of defects in the semiconductor wafer of FIG. 2.

FIG. 10A is an enlarged fragmentary view showing three stripe regions inthe lower left-hand corner of the semiconductor wafer of FIG. 9.

FIG. 10B is an enlarged, not-to-scale view of the stripe regions ofFIGS. 9 and 10A that shows the raster scan path followed by the scanningmechanism of FIG. 9 relative to a light sensitive detector to detectdefect images in a defect image field.

FIG. 11 is a diagram showing an array of pixel elements in the defectimage field under tenfold magnification and an array of light detectingelements of a charge-coupled device used in the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram of a first preferred embodiment of aninspection system 10 of the present invention that is designed to detectsemiconductor wafer defects having a diameter of about one-quartermicron or larger in the presence of a periodic structure comprising manyredundant circuit patterns. FIG. 2 is a diagram of a semiconductor wafer12 of the type inspection system 10 is designed to inspect for defects.Wafer 12 includes a regular array of normally identical dies 14 of whicheach has at least about twenty redundant circuit patterns 16 along eachof the X-axis 18 and Y-axis 20. Each die 14 is typically of square shapewith about 3 millimeter sides. FIGS. 3A-3C are photographs of anexemplary single die 14 showing highly repetitive circuit pattern withinsuch element for consecutively increasing magnifications. Although theyare of rectangular shape as shown in FIGS. 3A-3C, circuit patterns 16are assumed for purposes of simplifying the following discussion to beof square shape with about 50 micron sides.

With reference to FIG. 1, inspection system 10 includes a laser source22 that provides a nearly collimated beam of 442.5 nanometermonochromatic light rays 24 that strike a lens 26 that converges thelight rays to a point 28 located in the back focal plane of lens 26. Thelight rays 30 diverging from focal point 28 strike a small mirror 32that is positioned a short distance from focal point 28 to reflect arelatively narrow circular beam of light toward a Fourier transform lenssection 34, which is shown in FIG. 1 as a single element but which isimplemented in five lens elements as will be further described below.Mirror 32 obscures a small region in the center of the Fourier transformplane defined by lens section 34. The size of the obscured region issufficiently small so that defect information, which is locatedeverywhere in the Fourier transform plane, is only insignificantlyblocked by mirror 32.

The effective center of Fourier transform lens section 34 is positioneda distance of slightly less than one focal length away from mirror 32 toprovide collimated light rays 36 that strike the patterned surface ofwafer 12. Wafer 12 is mounted in a chuck 38 that constitutes part of atwo-dimensional translation stage 40. Wafer 12 is positioned in theobject or front focal plane 42 of lens section 34, and the collimatedlight rays 36 illuminate the patterned surface of wafer 12. Thecollimated light rays 36 illuminate a 20 millimeter diameter area of thesurface of wafer 12. The light rays 44 diffracted by the illuminatedarea of wafer 12 pass through lens section 34 and form the Fouriertransform pattern of the illuminated wafer surface in the back focalplane 46 of lens section 34.

The Fourier transform pattern comprises an array of bright spots oflight that are distributed in back focal plane 46 in a predictablemanner. The 20 millimeter diameter illuminated area of wafer 12 providesa Fourier transform pattern of sufficient accuracy because it is formedfrom many redundant circuit patterns. The design of lens section 34 is,however, such that it has only a 3 millimeter object field diameter toform in the image plane 60 an essentially aberration-free image ofdefects in the semiconductor wafer. An entire die can be inspected fordefects because translation stage 40 moves the die through theilluminated area. Therefore, a relatively large area of wafer 12 isilluminated to develop an accurate Fourier transform pattern of theredundant circuit patterns, but .[.a.]. .Iadd.an inverse Fouriertransform .Iaddend.lens .Iadd.section 54 .Iaddend.of relatively smallobject field diameter collects the light diffracted by .Iadd.the centralportion of .Iaddend.the illuminated area to minimize the introduction ofaberrations into the .[.Fourier transform pattern.]. .Iadd.image.Iaddend.as it is formed.

A previously fabricated spatial filter 50 is positioned in the plane 46of the Fourier transform pattern. Spatial filter 50 can be fabricated insitu by exposing a recording medium, such as a photographic plate, tolight diffracted by all of the dies 14 of wafer 12. This can beaccomplished with nonerror-free wafer 12 because the defect informationcarried by light of relatively low intensity would not expose thephotographic plate while Fourier transform information carried byrelatively high intensity light exposes the photographic plate. Spatialfilter 50 can also be fabricated in accordance with known computergeneration techniques.

Spatial filter 50 blocks the spatial frequencies of the error-freeFourier transform of the illuminated dies 14 of wafer 12 but allows thepassage of light originating from possible defects in, and lightdiffracted by other wafer dies proximally located to, such dies. Thedefect-carrying light rays 52 not blocked by spatial filter 50 strike.[.an.]. inverse Fourier transform lens section 54, which is shownschematically as a single lens but includes four lens elements as willbe further described below. Inverse Fourier transform lens section 54performs the inverse Fourier transform on the filtered light pattern ofthe illuminated wafer dies 14. Lens section 54 is positioned a distanceof one focal length away from back focal plane 46 of lens section 34.The elements of lens sections 34 and 54 are aligned along the same opticaxis 48, and translation stage 40 moves the wafer dies 14 across theoptic axis 48.

A photodetector array 58 is centrally positioned about optic axis 48 inan image plane 60 and receives the image of the defects present in theon-axis portion wafer die 14. Image plane 60 is located in the backfocal plane of lens section 54. The magnification of lens section 54 isof an amount that approximately matches the resolution limit of theimage to the pixel size of photodetector array 58. In particular,photodetector array 58 has a light sensitive surface 62 whose dimensionsare about 10 millimeters×10 millimeters within the 30 millimeterdiameter image area. A tenfold magnification is, therefore, the properamount to detect defects in the 3 millimeter diameter area of theon-axis wafer die 14.

To inspect the entire patterned surface of wafer 12, translation stage40 sequentially moves each portion of the die 14 of wafer 12 to opticaxis 48 for illumination by the light emanating from the light source22. The area of light sensitive surface 62 of the stationaryphotodetector array 58 limits the amount of light detected to that of aportion of the image corresponding to only the wafer die 14 centeredabout optic axis 48. The image information corresponding to any portionof illuminated off-axis wafer dies 14 cannot, therefore, reachphotodetector array 58. The movement of translation stage 40 iscontinuous in a stripe-to-stripe raster fashion to implement a timedelay integration technique for collecting the defect image informationfor each die 14 on the patterned surface of wafer 12.

The Fourier transform lens section 34 and inverse Fourier transform lenssection 54 are designed as part of one optical system 68 andcollectively have ten elements as shown in FIG. 4. The design of opticalsystem 68 is complicated by the stringent requirements for two importantdesign parameters, namely, the minimum spot diameter "d₁ " in Fouriertransform plane 46 and minimum spot diameter "d₂ " in image plane 60. Asmall minimum spot diameter in Fourier transform plane 46 is required toresolve the bright spots produced in such plane by circuit patterns 16of the largest expected size. If pattern 16 is of square shape, therequired spot diameter d₁ satisfies the following expression:

    d.sub.1 <<λf.sub.1 /c,

where λ is the wavelength of light emanating from laser source 22, f₁ isthe effective focal length of lens section 34, and c is the length of aside of the square pattern 16. A spot diameter d₁ of 20 microns can berealized for c<300 microns.

A small minimum spot diameter in image plane 60 determines the smallestpossible detectable defect size. The minimum spot diameter d₂ isdetermined by the cooperation of lens sections 34 and 54, and the imagemagnification "m." A defect of a diameter greater than d₂ /m can bemeasured from the spatial spread of its image. A defect of a diameterless than d₂ /m, i.e., a subresolution defect, has an image spread thatequals d₂ but has an image intensity that decreases quadradically withincreasing diameter. To detect subresolution defects, inspection system10 must be designed to achieve substantially lower electronic or opticalnoise. The design parameter is achievable with the preferred embodimentsof inspection system 10 for d₂ =10 microns, m=10, and d₂ /m=1 micronover the 30 millimeter diameter image field.

With reference to FIGS. 4 and 5, optical system 68 is a neardiffraction-limited optical system that accepts light diffracted into a±15°-20° telecentric cone, forms the periodic structure of the Fouriertransform of the object (i.e., the redundant circuit patterns of a waferdie 14), and produces a detectable image of one-quarter micron or largerdiameter defects. The design of lens section 34 is of asymmetriccharacter to form nearly diffraction-limited light pattern at theFourier transform plane 46. The reason for the asymmetry is that thediffraction angle of optical system 68 is relatively large (±15°-20°)and the 3 millimeter diameter of the surface to be imaged is moderatelylarge. Lens section 54 requires a relatively long focal length f₂ toachieve the 10X image magnification. The design of lens section 54 is ofasymmetric character and the entrance pupil is positioned closer to thefront lens element 72 of lens section 54 to balance residual aberrationsintroduced by lens section 34 and to minimize the length of opticalsystem 68 and the diameters of the lens elements incorporated in it.

The plane of lens section 54 is positioned nearly in contact withFourier transform plane 46 to provide a compact signal filteringarrangement. Sufficient space is required between lens section 54 andFourier transform plane 46 to introduce the illuminating beam throughthe Fourier transform plane, and to accommodate the mechanical structurethat supports spatial filter 50. Optical system 68 is designed so thatthe image will be an inverted copy of the object with a magnification of-f₂ /f₁, where f₂ =600 millimeters, which is the effective focal lengthof lens section 54, and f₁ =60 millimeters, which is the effective focallength of lens section 34. Therefore, the magnification "m" equals 10.

Lens section 34 is designed to meet the following two performancerequirements. The first and most demanding requirements is that lightdiffracted into a ±15°-20° telecentric cone from any point on a 3millimeter diameter object placed at the object or front focal plane 42is collimated with sufficiently small light ray aberrations to permitthe ultimate formation of a near diffraction-limited image with verylittle geometric distortion. The second requirement is that plane wavespropagating through a 20 millimeter object diameter over a range of±15°-20° have minimum vignetting and produce a light pattern at Fouriertransform plane 46 of less than 20 micron resolution spot diameter.

Residual aberrations introduced by lens section 34 into lens section 54are magnified and are, therefore, nearly impossible to eliminate bycompensating aberrations in lens section 54. The design requirementsare, therefore, that lens section 34 at Fourier transform plane 46 be 1)isoplanatic (i.e., the aberrations remain constant over a small sectionof the Fourier transform image field so that the lens is a linear,shift-invariant filter of spatial frequencies), (2) essentiallyaplanatic (i.e., free from spherical and coma aberrations), and (3)essentially anastigmatic (i.e., having a flat field with noanastigmatism) for incident plane waves over a ±15°-20° diffractionangle and over a 20 millimeter entrance pupil diameter.

To produce a nearly diffraction-limited image at image plane 60, lenssection 34 must also produce a plane wave for a point object placedwithin a 3 milllimeter diameter region in the object or front focalplane 42 of the lens. This requires that the chief rays over the±15°-20° diffraction angle range must be forced to be telecentric (i.e.,parallel to optic axis 48) during the design of lens section 34 so thatresidual aberrations presented to lens section 54 will be very small andcompensatible.

The design approach of lens section 34 assumes that it receives lightemanating from an infinitely distant object with a ±15°-20° subtense andan entrance pupil placed at front focal plane 42. The Fourier transformlight pattern is, therefore, located at the back focal plane 46 of thelens.

In particular, lens section 34 includes five elements positioned alongand centered about optic axis 48. Element 72 is a double-convex lens andelement 74 is a positive meniscus lens that are positioned close to theentrance pupil of lens system 68 to control spherical aberrations. Adouble-concave lens 76 controls the field curvature, and double-convexlens 78 and positive meniscus lens 80 are positioned in the convergingbeam to control astigmatism. To achieve the aberration control driven bythe ±15°-20° range of diffraction angles, lens section 34 requires thefive elements 72, 74, 76, 78, and 80, which are constructed from glassof a high refractive index.

The design of lens section 54 balances the residual spherical and comaaberrations introduced by lens section 34 when object points are placedat its front focal plane 46 within a 3 millimeter object diameter. Adouble-convex lens element 70 and a double-concave element 84 arepositioned close to Fourier transform plane 46 to cancel the residualspherical aberrations introduced by lens section 34. A negative meniscuslens element 86 cancels the coma aberrations introduced by lens section34. A positive meniscus lens element 88 of weak positive powerdistributes the refractive power of lens section 54 so that elements 70,84, and 86 can introduce the right amount of spherical and comaaberrations to cancel residual aberrations from lens section 34. Lenselement 88 also helps correct the astigmatism in the image plane 60. Aplano-convex lens element 90 of weak positive power is positionedreasonably close to image plane 60 to control geometric distortion inthe image. The positive power of element 90 diminishes, however, thecorrection of field curvature and astigmatism. The image quality atimage plane 60 marginally meets the design objectives because of thelimitations imposed by such imbalance of field curvature and astigmatismand the existence of higher order residual spherical aberrationsintroduced by lens section 34.

Tables I and II summarize the design specifications for and the spacingbetween adjacent elements of optical system 68. Table I includes theprescription for the elements of lens section 34 and spatial filter 50,and Table II includes the prescription for the elements of lens section54. The surfaces a-w correspond in general to lettered surfaces in FIG.5, in which surface "a" corresponds to the object plane 42 and surface"w" corresponds to the image plane 60. Surface l₁ and l₂ correspond tospatial filter 50. In each instance, the radius and aperture diameter ofthe surface are given and the shape of each surface is spherical, exceptfor surfaces a, l₁, l₂, v, and w, which are flat. A positive radius fora surface indicates the center of curvature is to the right in thedrawing, and a negative radius indicates the center of curvature is tothe left in the drawing (FIG. 5). Dimensions are given in millimeters,and the axial distance to the next surface is measured from left toright in FIG. 5.

                                      TABLE I                                     __________________________________________________________________________          RADIUS OF                                                                             AXIAL DISTANCE                                                                            APERTURE     GLASS                                  SURFACE                                                                             CURVATURE                                                                             TO NEXT SURFACE                                                                           DIAMETER     TYPE                                   __________________________________________________________________________    a     INFINITY                                                                              25.4000     3.000                                               b     29.4604 7.6200      16.2210      SF4                                    c     -1550.4129                                                                            0.8196      16.4346      SF4                                    d     -59.1204                                                                              6.3500      20.0000      SF4                                    e     -42.6729                                                                              7.3181      17.3541      SF4                                    f     -25.8759                                                                              3.8100      25.4000      SF8                                    g     31.6136 9.2731      25.4000      SF8                                    h     295.2013                                                                              12.7000     27.1251      SF4                                    i     -46.9393                                                                              1.1064      31.8356      SF4                                    j     76.1677 12.7000     33.6826      SF4                                    k     1792.0762                                                                             23.0610     38.1000      SF4                                    .sub. l.sub.1                                                                       INFINITY                                                                              1.5000      38.1000                                                                           (10.160 obscured)                                                                      K5                                     .sub. l.sub.2                                                                       INFINITY                                                                              12.7000     38.1000                                                                           (10.160 obscured)                                                                      K5                                     __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________          RADIUS OF                                                                             AXIAL DISTANCE                                                                            APERTURE                                                                              GLASS                                       SURFACE                                                                             CURVATURE                                                                             TO NEXT SURFACE                                                                           DIAMETER                                                                              TYPE                                        __________________________________________________________________________    m     44.9835 12.7000     33.6063 K5                                          n     -108.6354                                                                             8.1484      31.5875 K5                                          o     -186.5238                                                                             9.5250      31.7500 SF4                                         p     56.7478 47.7058     31.7500 SF4                                         q     -12.2051                                                                              9.5250      22.0000 SK11                                        r     -16.9977                                                                              194.7294    27.3846 SK11                                        s     70.7985 12.7000     37.8796 SILICA                                      t     88.1911 291.8083    38.1000 SILICA                                      u     203.8331                                                                              6.3500      33.0126 BK7                                         v     INFINITY                                                                              55.9925     32.6558 BK7                                         w     INFINITY                                                                              0.0000      27.8444                                             __________________________________________________________________________

FIG. 6 is a schematic diagram of a second preferred embodiment of aninspection system 100 of the present invention that is designed todetect defects in semiconductor wafers of the kind inspected by theabove-described inspection system 10. Inspection system 100 is designedto meet approximately the same performance specifications as those ofinspection system 10. Inspection system 100 includes a folded Fouriertransform optical system that includes an analytically defined Fouriertransform spectral pattern that is inscribed in a liquid crystal layerspatial filter 102. The liquid crystal spatial filter 102 scatters lightof the spatial frequencies associated with the regular and periodicstructure of, and reflects light of spatial frequencies associated withdefects in, the patterned surface of semiconductor wafer 12. Thatspatial filter 102 reflects light originating from possible defects inwafer 12 dictates the folded Fourier optical configuration of inspectionsystem 100.

With reference to FIG. 6, inspection system 100 includes a laser source104 that provides a nearly collimated beam of 442.5 nanometermonochromatic light rays 106 that strike a lens 108 that converges thelight rays to a point 110 located at the center of the aperture of apinhole spatial filter 112. The beam of light emitted by laser 104 islinearly polarized in the plane of FIG. 6. The light rays 114 divergingfrom focal point 110 strike a polarizing beam splitter 116 of the platetype which reflects light rays polarized in the plane perpendicular tothe plane of FIG. 6 but transmits light rays polarized in the plane ofFIG. 6. A quarter-wave plate 118 receives and imparts circularpolarization to the light rays 114 transmitted through beam splitter116. The circularly polarized light rays 114 exiting quarter-wave plate118 are confined to a relatively narrow circular beam and propagatetoward a Fourier transform lens section 120, which is shown in FIG. 6 asa single element but which is implemented in five lens elements as willbe further described below.

The effective center of Fourier transform lens section 120 is positioneda distance of one focal length away from pinhole spatial filter 112 toprovide collimated circularly polarized light rays 122 that strike thepatterned surface of wafer 12. Wafer 12 is mounted in chuck 38 ontranslation stage 40, and the collimated light rays 122 illuminate a 20millimeter diameter of the surface of wafer 12, which is positioned in afront focal or object plane 124 in a manner analogous to that describedabove for inspection system 10.

Circularly polarized light rays 126 diffracted by the illuminated areaof wafer 12 propagate through lens section 120 and quarter-wave plate118, which develops linearly polarized light rays 128 in a directionperpendicular to the plane of FIG. 6. The light rays 128 reflect offpolarizing beam splitter 116 toward a quarter-wave plate 130, whichimparts circular polarization to light rays 126. The circularlypolarized light rays 132 exiting quarter-wave plane 130 strike the laserabsorbing layer 134 of spatial filter 102. The light rays 132 form theFourier transform pattern of the illuminated wafer surface in the backfocal plane 136 of lens section 120.

Spatial filter 102 blocks by absorption the spatial frequencies of theerror-free Fourier transform of the illuminated dies 14 of wafer 12 butallows by reflection the passage of light originating from possibledefects in such die. Spatial filter 102 differs from spatial filter 50in two major respects. First, the error-free Fourier transform patternis inscribed in a liquid crystal layer for spatial filter 102 and in aphotographic emulsion deposited on a photographic plate for spatialfilter 50. Second, spatial filter 102 is of a reflective type, andspatial filter 50 is of a transmissive type.

The defect-carrying circularly polarized light rays 138 reflected byspatial filter 102 propagate through quarter-wave plate 130, whichalters the circular polarization of the light rays and develops linearlypolarized light rays 140 whose polarization direction is in the plane ofFIG. 6. The light rays 140 propagate through beam splitter 116 andstrike an inverse transform Fourier lens section 142, which is shownschematically as a single lens but includes five lens elements as willbe further described below. Inverse Fourier transform lens section 142performs the inverse Fourier transform on the filtered light pattern ofthe illuminated wafer dies 14. Lens section 142 is positioned a distanceof one focal length away from back focal plane 136 of lens section 120.The elements of lens sections 120 and 142 are aligned along or aredecentered relative to the same optic axis 144, which is folded into twosections 146 and 148 at the plane of beam splitter 116. Translationstage 40 moves the wafer dies 14 across section 146 of optic axis 144.

Photodetector array 58 is centrally positioned about optic axis 144 inan image plane 150 and receives the image of the defects present on theon-axis position wafer die 14. Image plane 150 is located in the backfocal plane of lens section 142. The magnification of lens section 54 isthe same as that of, and is determined for the same reasons as thosedescribed for, lens section 54 of inspection system 10. The inspectionof the entire patterned surface of wafer 12 is accomplished in a manner.[.analagous.]. .Iadd.analogous .Iaddend.to that described below forinspection system 10.

FIG. 7 is a cross sectional view of spatial filter 102, whichconstitutes a laser smectic light valve in which the error-free Fouriertransform pattern of wafer 12 is inscribed. The laser smectic lightvalve of the type employed in the present invention is described inKahn, Frederic J., "LARGE AREA, ENGINEERING DRAWING QUALITY DISPLAYSUSING LASER ADDRESSED SMECTIC LIQUID CRYSTAL LIGHT VALVES," AutomationTechnology Institute Conference, Montreal, Canada, February 1987.

With reference to FIG. 7, spatial filter 102 comprises a pair ofspaced-apart glass substrates 160 and 162 that capture a smectic liquidcrystal material 164 between them. A laser absorber layer 166 is appliedto the inner surface of glass substrate 160. A reflector electrode 168is applied to the inner surface of laser absorber layer 166, and atransport electrode 170 is applied to the inner surface of glasssubstrate 162. Director alignment layers 172 are applied to the innersurface of reflector electrode 168 and the inner surface of transparentelectrode 170. The directors 174 of the liquid crystal material 164contained within the cell have the layered parallel ordering of thesmectic phase. The error-free Fourier transform pattern is inscribed inspatial filter 102 in the following way.

A narrowly focused writing laser beam 176 propagates through glasssubstrate 160 and is focused on laser absorber layer 166, which absorbsthe incident laser light and converts it into heat. The heat rapidlydiffuses into a localized volume of liquid crystal material 164, raisingthe temperature of such localized volume by a sufficient amount to heatit above a critical transition temperature. The temperature increase isapproximately several degrees Centigrade.

Whenever the temperature of liquid crystal material 164 exceeds thecritical transition temperature, the directors 174 thereof no longerhave the layered parallel ordering of the smectic phase shown in FIG. 7but have a random ordering that is characteristic of an ordinaryisotropic liquid. Whenever the focused laser beam is extinguished ormoved to another writing location, the previously exposed localizedvolume cools very rapidly back to its ambient operating temperature, theheat diffusing into glass substrates 160 and 162. Glass substrates 160and 162 are typically 100 to 500 times thicker than liquid crystalmaterial 164, which forms a layer of approximately 13 microns inthickness. Liquid crystal material 164 cools at a rate such that thereis insufficient time for directors 174 to reorient to the uniformlyordered smectic configuration. Directors 174 retain their unorderedcharacteristic until they undergo an erasing or editing procedure.

The regions of spatial filter 102 heated by the writing laser beamscatter incident light, and the unheated regions of spatial filter 102do not scatter light. The written regions scatter incident lightpropagating in the direction 178 within spatial filter 102 when it isilluminated. The light is scattered so that it is not collected by lenssection 142 (FIG. 6). The unwritten regions of spatial filter 102 appearto act like a mirror when the surface of spatial filter 102 isilluminated.

The entire filter pattern can be erased by applying an AC voltagebetween the transparent electrode 170 on the inner surface of glasssubstrate 162 and the reflector electrode 168 on laser absorber layer166. The resultant electric field across liquid crystal material 164causes directors 174 to align parallel to the applied field and hencenormal to the surfaces of glass substrates 160 and 162 to provide thelight nonscattering surface.

The error-free Fourier transform can be inscribed into spatial filter102 by means of a laser-based scanning system that scans laser beam 176across the surface of glass substrate 160 to illuminate the appropriateareas of laser absorber layer 166 to form the written regions or lightscattering spots corresponding to the Fourier transform pattern.

The Fourier transform lens section 120 and inverse Fourier transformlens section 142 are designed as part of one optical system 200 andcollectively have ten elements as shown in FIG. 8. Lens section 120 isdesigned to meet the following two performance requirements. The firstrequirement is that light diffracted into a ±15°-20° telecentric conefrom any point on a 3 millimeter diameter object placed at object orfront focal plane 124 be collimated with sufficiently small light rayaberrations to permit the formation of a near diffraction-limited imagewith very little geometric distortion. The second requirement is that along back focal length be used to permit folding the system at the planeof beam splitter 116. The design of optical system 200 is complicated bythe stringent requirements for the minimum spot diameter "d₁ " inFourier transform plane 136 and minimum spot diameter "d₂ " in imageplane 150. The design parameters for spot diameters d₁ and d₂ and forthe image magnification "m" are the same as those described forinspection system 10.

To achieve a back focal length of large dimensions, lens section 120requires the five elements 202, 204, 206, 208, and 210.

Lens section 120 is of a Berthele eyepiece form because it produces aback focal length of large dimension and accommodates a large relativeaperture. Elements 204 and 206 are negative meniscus lenses that providestrong negative power required at the input of the Berthele eyepiece.

Lens section 120 forms the Fourier transform pattern from an incidentplane wave and forms in cooperation with lens section 142 an image oftenfold magnification of wafer 12 placed at its front focus 124. Thisdual function places severe constraints on the design of lens section120 because the chief rays of the ray fans associated with thediffracted energy that produces the Fourier transform pattern become therays that construct the axial image point of the magnified image. Underthese conditions the magnified image will exhibit very strong sphericalaberration whenever the chief rays are not exactly parallel to the opticaxis 144 (i.e., telecentric). If the chief rays do not intercept Fouriertransform plane 136 in accordance with the sine relationship (i.e., theintercept height equals the sine of the diffraction angle times thefocal length of lens section 120), the magnified image would exhibitvery strong coma aberration. The magnitude of the coma and sphericalaberrations if uncorrected in lens section 120 would be impossible tocompensate in lens section 142.

The residual spherical aberration is corrected by a flat plate 212having a spherical aberration correcting surface positioned downstreamof beam splitter 116. Aspheric corrector plate 212 eliminates nearly allof the spherical aberration, but its location and form introduces amodest amount of coma to the magnified image which is removed by lenssection 142.

Lens section 142 consists of five elements 214, 216, 218, 220, and 222that are preferably mounted in a long barrel. Elements 214, 216, and 218operate as a triplet, which is located immediately after asphericcorrector plate 212. The bending of elements 214, 216, and 218 primarilycorrects for coma introduced by aspheric corrector plate 212, and theirpower distribution and glass types are chosen to control the Petzvalfield curvature. Elements 220 and 222 correct the residual astigmatismin the system.

Tables III and IV summarize the design specifications for and thespacers between adjacent elements of optical system 220. Table IIIincludes the prescription for the elements of lens section 120,quarter-wave plates 118 and 130, beam splitter 116, and spatial filter102 and aspheric corrector plate 212; and Table IV includes theprescription for the elements of lens section 142.

                                      TABLE III                                   __________________________________________________________________________          RADIUS OF                                                                             AXIAL DISTANCE                                                                            APERTURE                                                                              GLASS                                       SURFACE                                                                             CURVATURE                                                                             TO NEXT SURFACE                                                                           DIAMETER                                                                              TYPE                                        __________________________________________________________________________    a     INFINITY                                                                              28.5039     20.0000                                             b     -30.8674                                                                              4.8260      32.7533 F8 SCHOTT                                   c     -38.4006                                                                              2.1198      36.6157 F8 SCHOTT                                   d     -28.1656                                                                              9.5250      36.6173 SF8 SCHOTT                                  e     -38.8061                                                                              0.5000      45.7214 SF8 SCHOTT                                  f     -72.2881                                                                              11.9413     48.9017 SF6 SCHOTT                                  g     -55.4938                                                                              0.5000      56.0021 SF6 SCHOTT                                  h     -635.9805                                                                             6.9850      60.1143 SF6 SCHOTT                                  i     -103.2759                                                                             0.5000      61.1565 SF6 SCHOTT                                  j     88.8919 7.2182      62.3434 SF6 SCHOTT                                  k     210.9940                                                                              2.8849      61.2792 SF6 SCHOTT                                  l     INFINITY                                                                              3.0000      61.1059 SILICA                                      m     INFINITY                                                                              33.5000     60.5615 SILICA                                      n     INFINITY                                                                              -34.0000    84.2419 REFLECTIVE                                  o     INFINITY                                                                              -3.0000     42.5162 SILICA                                      p     INFINITY                                                                              0.0000      41.9718 SILICA                                      q     INFINITY                                                                              -5.0000     41.9718 SILICA                                      r     INFINITY                                                                              5.0000      41.0657 SILICA                                      s     INFINITY                                                                              0.0000      41.0657 REFLECTIVE                                  t     INFINITY                                                                              5.0000      41.0657 SILICA                                      u     INFINITY                                                                              0.0000      41.0657 SILICA                                      v     INFINITY                                                                              3.0000      41.1143 SILICA                                      w     INFINITY                                                                              34.0000     41.1509 SILICA                                      x     INFINITY                                                                              5.0000      59.5915 BK7 SCHOTT                                  y     INFINITY                                                                              34.3000     64.8796 BK7 SCHOTT                                  z     A(1).sup.*                                                                            5.0000      42.4129 SF1 SCHOTT                                  aa    INFINITY                                                                              1.5000      42.6606 SF1 SCHOTT                                  __________________________________________________________________________     *(A1) = Ay.sup.4 +  By.sup.6 + Cy.sup.8 + Dy.sup.10                           where A = -7.37706 × 10.sup.-7 : B = -5.25329 × 10.sup.-10 :      = 5.10552 × 10.sup.-13 : D = -1.14906 × 10.sup.-15           

                                      TABLE IV                                    __________________________________________________________________________          RADIUS OF                                                                             AXIAL DISTANCE                                                                            APERTURE                                                                              GLASS                                       SURFACE                                                                             CURVATURE                                                                             TO NEXT SURFACE                                                                           DIAMETER                                                                              TYPE                                        __________________________________________________________________________    bb    44.7640 5.0800      43.2497 SF6 SCHOTT                                  cc    50.2426 9.2819      41.4749 SF6 SCHOTT                                  dd    -47.1953                                                                              3.8100      41.4747 SILICA                                      ee    -80.9227                                                                              123.9232    42.9293 SILICA                                      ff    -72.3297                                                                              6.3500      57.3855 SF6 SCHOTT                                  gg    -61.9630                                                                              311.8775    59.4953 SF6 SCHOTT                                  hh    43.6306 5.0800      38.8543 SILICA                                      ii    49.5340 13.3624     37.3900 SILICA                                      jj    -36.1991                                                                              5.0800      36.7221 SF6 SCHOTT                                  kk    -42.2545                                                                              275.6579    39.0828 SF6 SCHOTT                                  ll    INFINITY                                                                              0.0000      30.5109                                             __________________________________________________________________________

The surface a-ll correspond in general to lettered sections in FIG. 8,in which surface "a" corresponds to the object plane 124 and surface"ll" corresponds to the image plane 150. Surfaces l-aa correspond toquarter-wave plates 118 and 130, beam splitter 116, spatial filter 102,and aspheric corrector plate 212. Certain surfaces have two letters, oneletter representing the surface struck by light propagating towardspatial filter 102 and the other letter representing the surface struckby light reflected by spatial filter 102. In each instance, the radiusand aperture diameter of the surface are given and the shape of eachsurface is spherical, except for surfaces a, l-y, aa, and ll, which areflat, and surface z, which is aspheric. A positive radius for a surfaceindicates the center of curvature is to the right in the drawing and anegative radius indicates the center of curvature is to the left in thedrawing (FIG. 8). Dimensions are given in millimeters, and the axialdistance to the next surface is measured in the positive direction fromleft to right in FIG. 8.

Beam splitter 116 undergoes decentering by 45° rotation in a planeperpendicular to that of FIG. 8. Aspheric corrector plate 212 undergoesdecentering by a -2.1642 millimeter displacement downwardly in thevertical direction in FIG. 8. A decenter defines a new coordinate system(displaced and/or rotated) in which subsequent surfaces are defined.Surfaces following a decenter are aligned along the local mechanicalaxis of the new coordinate system. The new mechanical axis remains inuse until changed by another decenter.

The technique for detecting defects is the same for both of inspectionsystems 10 and 100; therefore, the following discussion is directed toinspection system 10 only for purposes for illustration. With referenceto FIGS. 1 and 9, the presence of defects in wafer 12 is determined bydetecting regions of light of intensities which exceed a predeterminedthreshold amount and which are positioned within an inspection area 250of a defect image field in image plane 60. Inspection area 250 includesthe space contained within the broken outline 252, which is defined bythe next adjacent sides of dies 14 to the perimeter of wafer 12 inFIG. 1. Since lens section 54 provides tenfold magnification, the defectimage field in image plane 60 has an area that is 100 times that ofinspection area 250.

The determination of the presence of defects in wafer 12 is accomplishedby partitioning it into stripe regions 254 of 1.0 millimeter in widthand moving translation stage 40 in a raster scan fashion so that the 20millimeter×20 millimeter spot emanating from laser 22 illuminatesstripe-by-stripe the entire surface of wafer 12. Since .[.t.]. .Iadd.it.Iaddend.is centrally disposed about optic axis 48 and has a 10millimeter×10 millimeter light sensitive surface 62, photodetector array58 detects only the aberration-free inverse Fourier transform lightpattern representing a 1.0 millimeter×1.0 millimeter on-axis illuminatedregion of wafer 12. (The tenfold magnification provided by lens section54 equalizes the dimensions of the 1.0 millimeter×1.0 millimeterilluminated wafer area and the corresponding 10 millimeter×10 millimeterdetected image area.)

Translation stage 40 comprises an X-Y positioning table that is capableof positioning wafer 12 in plane 42 for illumination by the 20millimeter×20 millimeter beam of light rays 36. A top or Y-stage 256 oftranslation stage 40 supports chuck 38 and moves wafer 12 along the Ydirection in plane 42. A bottom or X-stage 258 of translation stage 40moves wafer 12 along the X direction in plane 42. One suitable type ofX-Y positioning table is a Model 8500 manufactured by KensingtonLaboratories, Inc. of Richmond, Calif.

A control circuit (not shown) for translation stage 40 keeps wafer 12moving at a constant speed as it positions each stripe region forillumination at optic axis 48. Translation stage 40 provides positioncoordinate information indicating the position of translation stage 40and the position of defects in the corresponding defect image in imageplane 60 relative to a known location on wafer 12. The detection ofimage defects is performed in accordance with a time delay integrationtechnique, which is described below.

FIGS. 10A and 10B are diagrams of, respectively, the outline of thelower left-hand corner of wafer 12 in FIG. 2 and an enlarged portionthereof to show stripe regions 254 and the raster scan path translationstage 40 travels along time. FIG. 11, which is an enlarged diagram ofthe portions of the stripe regions of FIG. 10B, shows the one-to-onecorrespondence between the dimensions of the light detecting elements260 comprising light sensitive surface 62 of photodetector array 58 andthe pixel elements 262 having the same dimensions of light detectingelements 260 because of the tenfold magnification by lens section 54.The photodetector array 58 has 206,336 light detecting elements arrangedin 403 rows and 512 columns, as described below.

With reference to FIGS. 10A, 10B, and 11, photodetector array 58 has anoptical window 264 through which light passes to be detected by it.Optical window 264 is a rectangle which has sides 266 and 268 thatdefine its length and sides 270 and 272 that define its width. Opticalwindow 264 is fixed generally centrally about optic axis 48. The motionof wafer 12 moves the defect image field, which represents a magnifiedversion of inspection area 250, past optical window 264. For purposes ofclarity, however, the following description is presented as thoughinspection area 250 of wafer 12 moves past control window 264.

In a normal scan operation, translation stage 40 moves wafer 12 pastoptical window 264 in the X direction so that side 272 of optical window264 is aligned with the segment 278 of inspection area 250. The movementof wafer 12 past optical window 264 defines along stripe 254 a pathsegment 274a that has an effective start location 276 and extends to theright in FIGS. 10B and 11. Sides 266 and 268 of optical window 264 areparallel to the Y direction and define the width 280 of a stripe region254 (three of which are shown in FIG. 9) which represents the portion ofinspection area 250 that moves in the X direction past photodetectorarray 58.

After segment 282 of inspection area 250 moves past side 266 of opticalwindow 264, translation stage 40 moves wafer 12 such that it describes aretrace path segment 284a which extends to the left in FIGS. 10A and 11to define a start location 286 for the scan of a second adjacent striperegion 254. During retrace, Y-stage 256 moves wafer 12 a distance equalto width 280 (i.e., 100 millimeter) of stripe region 254, and X-stage258 moves wafer 12 a distance equal to the length of path segment 274a.After retrace, translation stage 40 moves wafer 12 along path segment274b in the X direction from start location 286 to traverse a secondstripe region 254 of width 280.

The above-described scanning and retrace procedure is repeated until theentire inspection area 250 traverses past optic axis 48. There are,however, differences in the lengths of the scan and retrace pathsegments to accommodate the differences in the dimensions in the Xdirection of inspection area 250.

With particular reference to FIG. 11, photodetector array 58 of thepreferred embodiment is an RCA Model 6220-004 charge-coupled device thatincludes an array 288 of light detecting elements 260 arranged in rows290 and columns 292. Array 288 has 403 rows and 512 columns of lightdetecting elements 260. A row 290 is defined as a group of elements 260arranged in a line perpendicular to the scan direction (i.e., in the Ydirection), and a column 292 is defined as a group of elements 260arranged in a line parallel to the scan direction (i.e., in the Xdirection). Each row 290 and each column 292 have lengths of 6.45millimeters and 10.24 millimeters, respectively. Each light detectingelement 260 is 16 microns in length and 20 microns in width. The widthof each one of stripe regions 254 is, therefore, equal to the totaldistance spanned by a row of 403 light detecting elements. Each one oflight detecting elements 260 receives through optical window 264 lightrays that emanate from the portion of inspection area 250 with which itis aligned and stores in its potential well a quantity of charge ormeasured energy value that corresponds to the intensity of the lightrays incident to it.

Each stripe region 254 of inspection area 250 is divided into an array294 of pixel elements 262, of which each has the same dimensions aslight detecting elements 260 of array 288 by the operation of lenssection 54. Pixel elements 262 of array 294 are arranged in rows 296 andcolumns 298, each row having 403 pixel elements and each column having anumber of pixel elements dictated by the length of the stripe region254. The presence of light in the stripe regions is detected by movinginspection area 250 past optical window 264 of photodetector array 58along each one of stripe regions 254 and acquiring the energy valuecorresponding to the intensity of light in each one of pixel elements262 in accordance with the following procedure.

X-stage 258 commences the scanning process by accelerating wafer 12 fromstart location 276 toward the left in the X direction until side 268 ofoptical window 54 is collinear with segment 300 of inspection area 250.X-stage 258 then moves wafer 12 at a nominally constant predeterminedspeed along stripe region 254.

Whenever light detecting elements 260 in the first row 290a of array 288align with pixel elements 262 in the first row 296a of array 294, thefollowing events take place. An electrical charge develops in thepotential well of each one of light detecting elements 260 in row 290a.The quantity of charge corresponds to the intensity of light present inthe pixel element. (The potential wells of light detecting elements 260have no charge accumulated in them prior to the scan of a stripe region254.) A row transfer clock signal that is applied to each row 290 ofarray 288 transfers the charge from each light detecting element 260 inrow 290a to the light detecting element in the same column 292 but inthe next adjacent or second row 290b. This transfer takes place aboutthe time the light detecting elements and the pixel elements are alignedwith each other. (Since X-stage 258 continuously moves wafer 12 alongstripe region 254, there is a negligible amount of image degradationthat results from aliasing between adjacent rows of the pixel elements.)After the transfer of charge from row 290a to row 290b, there exists noaccumulated charge in the potential wells of light detecting elements260 in row 290a.

Whenever light detecting elements 260 in second row 290b align withpixel elements 262 in the second row 296b of array 294, the followingevents take place. An electrical charge develops in the potential wellof each light detecting element 260 in rows 290a and 290b. The quantityof charge developing in each one of the light detecting elements 260 inrow 290b is added to the charge previously transferred to it. Thequantity of charge in the light detecting elements 260 in row 290brepresents, therefore, two energy values corresponding to the intensityof light present in a pixel element 262 in each column of row 296a ofarray 294. The row transfer clock signal transfers the charge from eachlight detecting element 260 in row 290b and row 290a to the lightdetecting element in the same column 292 but in the next adjacent thirdrow 290c and second row 290b, respectively.

The above-described procedure of (1) acquiring in a light detectingelement 260 in a row 290 an energy value corresponding to the intensityof light in a pixel element 262 with which the light detecting elementis aligned and (2) transferring the energy value to the light detectingelement 260 in the same column 292 but in the next adjacent row 290 withwhich the pixel element 262 has not previously been aligned is repeatedfor 255 cycles of the row transfer clock signal.

When 255 such row-to-row transfers have been completed, the lightdetecting elements in the 256th or last row 290d of array 288 align withthe pixel elements 262 in first row 296a of array 294. The 255previously accumulated energy values for each pixel element 262 in firstrow 296a are added to the 256th energy value acquired by each lightdetecting element 260 in last row 290d. Prior to the occurrence of the256th row transfer clock signal, energy values accumulated in the 512light detecting elements 260 corresponding to the pixel elements 262 inrow 296a are read out serially by a high-speed data transfer clocksignal. The accumulated energy values for pixel elements 262 areconverted to a digital format and processed by a threshold detector todetermine whether the amount of light present in each pixel element 262indicates the presence of a defect in a corresponding location in wafer12.

Upon the occurrence of the 256th cycle of the row transfer clock signal,the 255 previously accumulated energy values for each pixel element 262in second row 296b are added to the 256th energy value acquired by eachlight detecting element 260 in last row 290d. Prior to the occurrence ofthe 257th cycle of the row transfer clock signal, the contents of the512 light detecting elements 260 corresponding to the pixel elements 262in row 296b are read out and processed as described above.

For each succeeding cycle of the row transfer clock signal, the scan ofstripe region 254 continues such that 256 energy values for each pixelelement 262 in a row 296 and a column 298 of array 294 are accumulatedin the light detecting element 260 in the corresponding column 292 androw 290d of array 288.

There are several general aspects of the accumulation of energy valuesthat characterize the above-described scanning process. First, each oneof the light detecting elements 260 in row 290a never accumulates morethan one energy value for any one of the pixel elements 262 with whichit becomes aligned. Second, the light detecting elements 260 in a row290 presently aligned with the pixel elements 262 in a particular row296 always have one more energy value accumulated in them than the lightdetecting elements 260 in the next adjacent row 290 that was previouslyaligned with the particular row 296 of pixel elements 262. Third, eachone of the light detecting elements 260 in row 290d accumulates 256energy values corresponding to the light present in the pixel element262 with which it is aligned.

After segment 282 of inspection area 250 travels completely past side266 of optical window 264, the scan of a stripe region 254 is completed,and the accumulated energy values of the pixel elements 262 in the lastrow 296d of array 294 have been read out from the light detectingelements 260 of the last row 290d of array 288. X-stage 258 decelerateswafer 12 to a stop at stop location 302. (In FIG. 11, optical window.[.54.]. .Iadd.264 .Iaddend.is shown in phantom for inspection area 250in this position.) X-stage 258 and Y-stage 256 retrace wafer 12 alongpath segment 284a to position start location 286 at optical window 264.The potential wells of light detecting elements 260 are cleared duringthis time in preparation for the scan of the next adjacent stripe region254. The scan and retrace of the second and succeeding stripe regions254 proceed as described above.

It will be obvious to those having skill in the art that many changesmay be made in the above-described details of the preferred embodimentof the present invention without departing from the underlyingprinciples thereof. For example, a photomask, instead of a semiconductorwafer, can be inspected for defects. .[.Ispection.]. .Iadd.Inspection.Iaddend.systems 10 and 100 would, however, have to be modified todirect the laser light for transmission through the photomask. As asecond example, a polarizing beam splitter of the cube type can besubstituted for the plate-type beam splitter 116 employed in inspectionsystem 100. A cube type beam splitter would reduce background noiseresulting from light reflection but would require a change in theprescription of lens system 200 to reduce spherical aberrationsintroduced by such a beam splitter. The scope of the present inventionshould be determined, therefore, only by the following claims.

We claim:
 1. In an imaging system that includes first and second lensespositioned along an optic axis, the first lens producing from a specimena spatial frequency spectrum whose frequency components can beselectively filtered and the second lens producing an image of defectspresent in the specimen, a method of detecting defects in a specimenthat includes an array of normally substantially identical dies, each ofthe dies having many redundant circuit patterns, comprising:illuminatingplural die circuit patterns; generating a light pattern representingsubstantially the Fourier transform pattern of the illuminated diecircuit patterns, the light pattern including intra-die interferencepattern information; positioning an optical filter to receive the lightpattern and to block spatial frequency components thereof, the opticalfilter having relatively transparent and relatively nontransparentportions, the relatively nontransparent portion conforming to theFourier transform pattern of an error-free reference patterncorresponding to the die circuit patterns; collecting spatial frequencycomponents not blocked by the optical filter to form an image of thedefects, the collected spatial frequency components corresponding to asmall number of die circuit patterns relative to the number of diecircuit patterns in the array of dies and residing in a spatial regionintercepting the optic axis; and processing unblocked intra-die spatialfrequency components to determine the location and size of a possibledefect in .[.the.]. .Iadd.a .Iaddend.die.
 2. The method of claim 1 whichfurther comprises changing the position of the specimen relative to theposition of the optic axis so that different ones of the die circuitpatterns are positioned within the spatial region intercepted by theoptic axis, thereby to process the intra-die spatial frequencycomponents of the different ones of the die circuit patterns.
 3. Themethod of claim 1 in which the processing of the unblocked intra-diespatial frequency components is accomplished by positioning a lightsensitive detector surface generally centrally about the optic axis, thelight sensitive detector surface having an area that is smaller than thesurface area of the image of the defects.
 4. The method of claim 1 inwhich the first and second lenses cooperate to receive light diffractedby, and provide an image from the spatial frequency componentscorresponding to, the illuminated die circuit patterns.
 5. The method ofclaim 4 in which the first lens comprises a first lens section of pluralelements and the second lens comprises a second lens section of pluralelements, the first and second lens sections forming a neardiffraction-limited lens system of asymmetric character.
 6. The methodof claim 1 in which the illuminating means emits nearly collimatedlight, the method further comprising:defining with respect to thespecimen plural adjacent stripes, each stripe including a series ofadjacent dies; moving the specimen and the collimated light relative toeach other along the length of each stripe to illuminate the die circuitpatterns in proximal position to the optic axis; and processing theunblocked intra-die spatial frequency components corresponding to thedie circuit patterns in proximal position to the optic axis.
 7. Themethod of claim 6 in which the specimen is movable and the collimatedlight remains fixed relative to the optic axis.
 8. The method of claim 1in which the relatively transparent and relatively nontransparentportions of the optical filter are developed by computer generationtechniques.
 9. The method of claim 1 in which the relatively transparentand relatively nontransparent portions of the optical filter aredeveloped by positioning a recording medium in the location of theFourier transform pattern and exposing the recording medium to lightpropagating from the specimen.
 10. The method of claim 1 in which thecollected spatial frequency components correspond to fewer than all ofthe illuminated die circuit patterns.
 11. In an imaging system thatincludes first and second lenses positioned along an optic axis, thefirst lens producing from a specimen a spatial frequency spectrum whosefrequency components can be selectively filtered and the second lensproducing an image of defects present in the specimen, a method ofdetecting defects in a specimen that includes an array of normallysubstantially identical dies occupying a first area of the specimen,each of the dies having many redundant circuit patterns,comprising:illuminating a second area of the specimen, the second areacontaining die circuit patterns and intercepting the optic axis;generating a light pattern representing substantially the Fouriertransform pattern of the illuminated die circuit patterns, the lightpattern including intra-die interference pattern information;positioning an optical filter to receive the light pattern and to blockspatial frequency components thereof, the optical filter havingrelatively transparent and relatively nontransparent portions, therelatively nontransparent portion conforming to the Fourier transformpattern of an error-free reference pattern corresponding to the diecircuit patterns; collecting spatial frequency components not blocked bythe optical filter to form an image of the defects; and processing onlyunblocked intra-die spatial frequency components to determine thelocation and size of a possible defect in .[.the.]. .Iadd.a.Iaddend.die.
 12. The system of claim 11 in which the size of the firstarea differs from that of the second area.
 13. The system of claim 12 inwhich the second area is substantially smaller than the first area. 14.The system of claim 12 in which the second area contains more than onedie.
 15. In an imaging system that includes first and second lensespositioned along an optic axis, the first lens producing from a specimena spatial frequency spectrum whose frequency components can beselectively filtered and the second lens producing an image of defectspresent in the specimen, a method of detecting defects in a specimenthat includes an array of normally substantially identical diesoccupying a first area of the specimen, each of the dies having manyredundant circuit patterns, comprising:illuminating die circuit patternsincluded within a second area of the specimen; generating a lightpattern representing substantially the Fourier transform pattern of theilluminated die circuit patterns, the light pattern including intra-dieinterference pattern information; positioning an optical filter toreceive the light pattern and to block spatial frequency componentsthereof, the optical filter having relatively transparent and relativelynontransparent portions, the relatively nontransparent portionconforming to the Fourier transform pattern of an error-free referencepattern corresponding to the die circuit patterns; collecting spatialfrequency components not blocked by the optical filter to form an imageof the defects, the collected spatial frequency components correspondingto fewer than all of the illuminated die circuit patterns; andprocessing the unblocked intra-die spatial frequency components todetermine the location and size of a possible defect in the die.
 16. Themethod of claim 15 in which the second area is substantially smallerthan the first area.
 17. In an imaging system that includes first andsecond lenses positioned along an optic axis, the first lens producingfrom a specimen a spatial frequency spectrum whose frequency componentscan be selectively filtered and the second lens producing an image ofdefects present in the specimen, a method of detecting defects in aspecimen that includes an array of normally substantially identicaldies, each of the dies having many redundant circuit patterns,comprising:illuminating plural die circuit patterns; generating a lightpattern representing substantially the Fourier transform pattern of theilluminated die circuit patterns, the light pattern including intra-dieinterference pattern information; positioning an optical filter toreceive the light pattern and to block spatial frequency componentsthereof, the optical filter having relatively transparent and relativelynontransparent portions, the relatively nontransparent portionconforming to the Fourier transform pattern of an error-free referencepattern corresponding to the die circuit patterns; collecting spatialfrequency components not blocked by the optical filter within a regionproximal to the optic axis to form an image of the defects; andprocessing unblocked intra-die spatial frequency components to determinethe location and size of a possible defect in .[.the.]. .Iadd.a.Iaddend.die, the processed spatial frequency components correspondingto a small number of die circuit patterns relative to the number of diecircuit patterns in the array of dies and lying in a spatial regionintercepting the optic axis.
 18. An optical system for detecting defectsin a specimen pattern of a type that includes an array of normallyessentially identical dies of which each has many redundant circuitpatterns and which occupy a first area of the specimen, the systemcomprising:illuminating means for illuminating a second area of thespecimen, the second area being occupied by plural die circuit patterns;pattern generating means for generating a light pattern representingsubstantially the Fourier transform pattern of the illuminated diecircuit patterns, the light pattern including intra-die interferencepattern information; optical filter means receiving the light patternfor blocking spatial frequency components thereof, the optical filtermeans having relatively transparent and relatively nontransparentportions, the relatively nontransparent portion conforming to theFourier transform of an error-free reference pattern corresponding tothe die circuit patterns; collecting means for collecting the spatialfrequency components not blocked by the optical filter means; andprocessing means for processing only the unblocked intra-die spatialfrequency components to determine the location and size of a possibledefect in the die.
 19. The system of claim 18 in which the illuminatingmeans emits nearly collimated light and which further comprisespositioning means for changing the position of the specimen relative tothe position of the collimated light so that different ones of the diecircuit patterns occupy the second area of the specimen illuminated bythe collimated light, thereby to process the intra-die spatial frequencycomponents of the different ones of the die circuit patterns.
 20. Thesystem of claim 18 in which the pattern generating means and thecollecting means comprises respective first and second lenses positionedalong an optic axis that intersects the second area of the specimenilluminated by the illuminating means.
 21. The system of claim 18 inwhich the pattern generating means and the collecting means compriserespective first and second lenses that cooperate to receive lightdiffracted by, and provide an image from the spatial frequencycomponents corresponding to, the illuminated die circuit patterns. 22.The system of claim 21 in which the first lens comprise a first lenssection of plural elements and the second lens comprises a second lenssection of plural elements, the first and second lens sections forming anear diffraction-limited lens system of asymmetric character.
 23. Thesystem of claim 21 in which the first lens comprises a first lenssection of plural elements and the second lens comprises a second lenssection of plural elements, the first lens section forming the Fouriertransform pattern and cooperating with the second lens section toprovide a magnified image of the defects in the illuminated die circuitpatterns.
 24. The system of claim 18 in which the pattern generatingmeans and the collecting means comprise a folded Fourier transformoptical system that receives light diffracted by, and provides an imagefrom the spatial frequency components corresponding to, the illuminateddie circuit patterns.
 25. The system of claim 24 in which the specimencomprises a semiconductor wafer.
 26. The system of claim 24 in which theoptical filter means comprises a liquid crystal layer.
 27. The system ofclaim 26 in which the relatively nontransparent portion of the liquidcrystal layer scatters light of the spatial frequencies incident to it.28. The system of claim 18 in which the optical filter means comprises aliquid crystal layer.
 29. The system of claim 28 in which the relativelynontransparent portion of the liquid crystal layer scatters light of thespatial frequencies incident to it.
 30. The system of claim 18 in whichthe optical filter means comprises exposed light sensitive material. 31.The system of claim 18 in which the optical filter means comprises alens assembly that has an aperture of at least ±15°.
 32. The system ofclaim 18 in which the Fourier transform light pattern represents theFourier transform image.
 33. The system of claim 18 in which thespecimen comprises a semiconductor wafer.
 34. The system of claim 18 inwhich the second area is substantially smaller than the first area. 35.The system of claim 18 in which the illuminating means emits nearlycollimated light and the processing means comprises a light sensitivedetector having a light sensitive surface positioned generally centrallyabout the optic axis.[.,.]. .Iadd.: .Iaddend.the light detectorincluding plural light detecting elements arranged in a first array ofrows and columns and defining in the light pattern plural adjacentstripe regions each of which includes plural pixel elements arranged ina second array of rows and columns, and each light detecting elementbeing operable to provide a measured energy value corresponding to theamount of light present in any one of the pixel elements, and the systemfurther comprising:positioning means for positioning the specimenrelative to the collimated light to scan the light detecting means alonga stripe region of the light pattern so that in succession each lightdetecting element in one column of the first array traverses andacquires an energy value corresponding to the amount of light present ina pixel element in one column of the second array; accumulating means toaccumulate a total energy value proportional to the sum of the energyvalues acquired for the pixel element by all of the light detectingelements in the one column of the first array; and means to determinefrom the total energy value whether the amount of light in the pixelelement represents a defect in the specimen subject.
 36. The system ofclaim 35 in which the light detector comprises a charge-coupled device.37. The system of claim 35 in which the collimated light remainsstationary and the positioning means scans each one of the striperegions across the light sensitive surface in a serial manner.
 38. Thesystem of claim 37 in which the positioning means continuously moveseach stripe region across the collimated light.
 39. The system of claim35 in which the first array has a first row and N total number of rowsand which further comprises position-detecting means for detecting theposition of the first array relative to the stripe region, theposition-detecting means cooperating with the accumulating means so thateach one of the light detecting elements in the first row of the onecolumn never accumulates more than one energy value for any one of thepixel elements of the second array with which it becomes aligned, andeach one of the light detecting elements in the Nth row of the onecolumn has accumulated N number of energy values for any one of thepixel elements with which it becomes aligned. .Iadd.
 40. In an imagingsystem that includes first and second lenses positioned along an opticaxis, the first lens producing from a specimen a spatial frequencyspectrum whose frequency components can be selectively filtered and thesecond lens producing an image of defects present in the specimen, amethod of detecting nonperiodic defects in a specimen that includes oneor more dies having many redundant circuit patterns,comprising:illuminating plural die circuit patterns; generating a lightpattern representing substantially the Fourier transform pattern of theilluminated die circuit patterns, the light pattern including intra-dieinterference pattern information; positioning an optical filter toreceive the light pattern and to block spatial frequency componentsthereof, the optical filter having relatively transparent and relativelynontransparent portions, the relatively nontransparent portionconforming to the Fourier transform pattern of an error-free referencepattern corresponding to the die circuit patterns; collecting spatialfrequency components not blocked by the optical filter to form an imageof the defects, the collected spatial frequency components correspondingto a small number of die circuit patterns relative to the number ofilluminated die circuit patterns and residing in a spatial regionintercepting the optic axis; and processing unblocked intra-die circuitpattern spatial frequency components to determine the presence of apossible nonperiodic defect in a die. .Iaddend. .Iadd.
 41. The method ofclaim 40 which further comprises changing the position of the specimenrelative to the position of the optic axis so that different ones of thedie circuit patterns are positioned within the spatial regionintercepted by the optic axis, thereby to process the intra-die spatialfrequency components of the different ones of the die circuit patterns..Iaddend. .Iadd.42. The method of claim 40 in which the processing ofthe unblocked intra-die spatial frequency components is accomplished bypositioning a light sensitive detector surface generally centrally aboutthe optic axis, the light sensitive detector surface having an area thatis smaller than the surface area of the image of the defects. .Iaddend..Iadd.43. The method of claim 40 in which the first and second lensescooperate to receive light diffracted by, and provide an image from thespatial frequency components corresponding to, the illuminated diecircuit patterns. .Iaddend. .Iadd.44. The method of claim 40 in whichthe illuminating means emits substantially collimated light, the methodfurther comprising:defining with respect to the specimen adjacentstripes for scanning through many redundant die circuit patterns; movingthe specimen and the collimated light relative to each other along thelength of each stripe to illuminate die circuit patterns in proximalposition to the optic axis; and processing the unblocked intra-diespatial frequency components corresponding to the die circuit patternsin proximal position to the optic axis. .Iaddend. .Iadd.45. The methodof claim 44 in which the stripe for scanning includes more than one areaof redundant die circuit patterns. .Iaddend. .Iadd.46. The method ofclaim 44 in which the stripe for scanning traverses through one or moredies. .Iaddend. .Iadd.47. An optical system for detecting nonperiodicdefects in a specimen pattern of a type that includes one or more dieshaving many redundant circuit patterns occupying a first area of thespecimen, the system comprising:illuminating means for illuminating asecond area of the specimen, the second area being occupied by pluralredundant die circuit patterns; pattern generating means for generatinga light pattern representing substantially the Fourier transform patternof the illuminated die circuit patterns, the light pattern includingintra-die interference pattern information; optical filter meansreceiving the light pattern for blocking spatial frequency componentsthereof, the optical filter means having relatively transparent andrelatively nontransparent portions, the relatively nontransparentportion conforming to the Fourier transform of an error-free referencepattern corresponding to the die circuit patterns; collecting means forcollecting the spatial frequency components not blocked by the opticalfilter means; and processing means for processing the unblockedintra-die spatial frequency components to determine the presence of apossible nonperiodic defect in a die. .Iaddend. .Iadd.48. The system ofclaim 47 in which the illuminating means emits substantially collimatedlight and in which the collecting means collects spatial frequencycomponents residing in a spatial region intercepting the optic axis, thespatial frequency components corresponding to a smaller number of diecircuit patterns relative to the number of illuminated die circuitpatterns and being in proximal position to the optic axis. .Iaddend..Iadd.49. The system of claim 47 in which further comprises positioningmeans for changing the position of the specimen so that different onesof the die circuit patterns are positioned within the second area,thereby to process the intra-die spatial frequency components of thedifferent ones of the die circuit patterns. .Iaddend. .Iadd.50. Thesystem of claim 47 in which the collecting means collects spatialfrequency components corresponding to fewer than all of the illuminateddie circuit patterns. .Iaddend. .Iadd.51. The system of claim 47 inwhich the illuminating means emits substantially collimated light and inwhich the pattern generating means and the collecting means compriserespective first and second lenses that cooperate to receive lightdiffracted by, and provide an image from the spatial frequencycomponents residing in a spatial region intercepting the optic axis andcorresponding to, a smaller number of die circuit patterns relative tothe number of illuminated die circuit patterns and positioned proximallyto the optic axis, the system further comprising:positioning means forchanging the position of the specimen relative to the position of thecollimated light to scan the specimen in stripes so that different onesof the die circuit patterns serially occupy the second area. .Iaddend..Iadd.52. The system of claim 51 in which the positioning means scans astripe that traverses one or more dies including one or more areashaving many redundant die circuit patterns. .Iaddend. .Iadd.53. Thesystem of claim 51 in which the first lens comprises a first lenssection of plural elements and the second lens comprises a second lenssection of plural elements, the first and second lens sections forming anear diffraction-limited lens system of asymmetric character. .Iaddend..Iadd. . The system of claim 51 in which the first lens comprises afirst lens section of plural elements and the second lens comprises asecond lens section of plural elements, the first lens section formingthe Fourier transform pattern and cooperating with the second lenssection to provide a magnified image of the defects in the illuminateddie circuit patterns. .Iaddend. .Iadd.55. In an imaging system thatincludes first and second lenses positioned along an optic axis, thefirst lens producing from a specimen a spatial frequency spectrum whosefrequency components can be selectively filtered and the second lensproducing an image of defects present in the specimen, a method ofdetecting nonperiodic defects in a specimen that includes one or moredies having many redundant die circuit patterns, comprising:illuminatinga small number of die circuit patterns relative to the number of diecircuit patterns in the specimen; generating a light patternrepresenting substantially the Fourier transform pattern of theilluminated die circuit patterns, the light pattern including intra-dieinterference pattern information; positioning an optical filter toreceive the light pattern and to block spatial frequency componentsthereof, the optical filter having relatively transparent and relativelynontransparent portions, the relatively nontransparent portionconforming to the Fourier transform pattern of an error-free referencepattern corresponding to the die circuit patterns; collecting spatialfrequency components not blocked by the optical filter to form an imageof the defects, the collected spatial frequency components residing in aspatial region intercepting the optic axis and corresponding to a smallnumber of die circuit patterns relative to the number of illuminated diecircuit patterns and being in proximal position to the optic axis; andprocessing the unblocked intra-die spatial frequency components todetermine the presence of a possible nonperiodic defect. .Iaddend..Iadd. . The method of claim 55 which further comprises changing theposition of the specimen relative to the position of the optic axis sothat spatial frequency components of different ones of the die circuitpatterns are collected within the spatial region intercepted by theoptic axis, thereby to process the intra-die spatial frequencycomponents of the different ones of the die circuit patterns. .Iaddend..Iadd.57. The method of claim 55 in which the illuminating means emitsnearly collimated light, the method further comprising:defining withrespect to the specimen adjacent stripes for scanning through manyredundant die circuit patterns; moving the specimen and the collimatedlight relative to each other along the length of each stripe toilluminate the circuit patterns; and processing the unblocked intra-diefrequency components corresponding to the circuit patterns in proximalposition to the optic axis. .Iaddend. .Iadd.58. The method of claim 57in which the stripe for scanning traverses through one or more dies..Iaddend.